Learn Swedish: Han måste hitta nyckeln till bilen. - He must find the key to the car.

While bilens nyckel is grammatically correct, Swedish strongly prefers using till for things that physically belong to or fit into another object. It sounds much more natural to use till for fixed relationships, like keys to cars, lids to jars, or doors to houses.

Why are both nyckel and bil in their definite forms here?

When showing possession with till, both nouns are usually in the definite form. Because we are talking about the specific key that belongs to the specific car, we must use nyckeln (the key) and bilen (the car).

Why isn't there an att (to) before the verb hitta?

Måste is a modal verb, which means it works as a helper verb. In Swedish, just like in English, modal verbs are followed directly by the infinitive form of the main verb without att. You say måste hitta, never måste att hitta.

Why is the definite form of nyckel spelled nyckeln instead of nyckelen?